U.S. patent number 4,337,170 [Application Number 06/114,566] was granted by the patent office on 1982-06-29 for catalytic steam reforming of hydrocarbons.
This patent grant is currently assigned to Union Carbide Corporation. Invention is credited to Andrija Fuderer.
United States Patent |
4,337,170 |
Fuderer |
June 29, 1982 |
Catalytic steam reforming of hydrocarbons
Abstract
The hot effluent from the catalytic steam reforming of a major
portion of a fluid hydrocarbon feed stream in the reformer tubes of
a primary reformer, or said effluent after secondary reforming
thereof, is mixed with the hot effluent from the catalytic steam
reforming of the remaining portion of the feed discharged from the
reformer tubes of a primary reformer-exchanger. The combined gas
steam is passed on the shell side of the reformer-exchanger
countercurrently to the passage of feed in the reformer tubes
thereof, thus supplying the heat for the reforming of the portion
of the feed passed through the reformer tubes of the
reformer-exchanger. At least about 2/3 of the hydrocarbon feed
stream is passed to the reformer tubes of said primary reformer,
heated by radiant heat transfer and/or by contact with combustion
gases, at a steam/hydrocarbon mole ratio of about 2-4/1. The
remainder of said feed stream is passed to the reformer tubes of
said reformer-exchanger at a steam/hydrocarbon mole ratio of about
3-6/1. The reformer shell of the reformer-exchanger is internally
insulated by a refractory lining or by use of a double shell with
passage of water or a portion of the feed material between the
inner and outer shells. There is no significant difference between
the pressure inside and outside of the reformer tubes of said
primary reformer-exchanger.
Inventors: |
Fuderer; Andrija (Antwerp,
BE) |
Assignee: |
Union Carbide Corporation
(Danbury, CT)
|
Family
ID: |
22356049 |
Appl.
No.: |
06/114,566 |
Filed: |
January 23, 1980 |
Current U.S.
Class: |
252/373; 422/628;
48/197R; 48/214R; 48/214A; 422/200 |
Current CPC
Class: |
C01B
3/384 (20130101); B01J 8/062 (20130101); C01B
2203/127 (20130101); C01B 2203/141 (20130101); C01B
2203/1294 (20130101); C01B 2203/043 (20130101); C01B
2203/0233 (20130101); C01B 2203/1241 (20130101); C01B
2203/0866 (20130101); C01B 2203/0894 (20130101); C01B
2203/0883 (20130101); C01B 2203/0811 (20130101); C01B
2203/0844 (20130101); C01B 2203/1011 (20130101); Y02P
20/129 (20151101); C01B 2203/1041 (20130101); C01B
2203/1247 (20130101); C01B 2203/0816 (20130101); C01B
2203/0283 (20130101); C01B 2203/1052 (20130101) |
Current International
Class: |
B01J
8/06 (20060101); B01J 8/02 (20060101); C01B
3/00 (20060101); C01B 3/38 (20060101); C01B
003/38 (); C01B 003/48 () |
Field of
Search: |
;252/373
;48/214R,214A,197R |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Mars; Howard T.
Attorney, Agent or Firm: Fritschler; Alvin H.
Claims
What is claimed is:
1. An improved process for the catalytic stream reforming of fluid
hydrocarbons comprising:
(a) catalytically reacting a major portion of a fluid hydrocarbon
feed stream with steam in catalyst-containing reformer tubes
positioned within a first primary reforming zone maintained at an
elevated temperature by radiant heat transfer and/or by contact
with combustion gases, the hot reformer tube effluent comprising a
first reformed gas mixture which is passed directly to the hot
discharge end of the reformer tubes in the second primary reforming
zone;
(b) catalytically reacting the remaining portion of said
hydrocarbon feed stream with steam at an elevated temperature in a
second primary reforming zone having catalyst-containing reformer
tubes positioned therein, the hot effluent from said tubes
comprising a second reformed gas mixture;
(c) combining the first and second reformed gas mixtures at the hot
discharge end of the reformer tubes of said second primary
reforming zone, thus forming a hot combined reformer gas stream
within said second primary reforming zone at the hot discharge end
of the reformer tubes therein;
(d) passing the hot combined reformer gas stream countercurrently
on the shell side of said second primary reaction zone, thereby
supplying heat to maintain said second primary reforming zone at an
elevated temperature; and
(e) withdrawing the thus-partially cooled combined reformer
effluent stream, comprising a combination of said first and second
reformed gas mixtures, from said second primary reaction zone,
whereby the desired overall steam reforming is accomplished at a
substantial reduction in hydrocarbon fuel consumption.
2. The process of claim 1 in which said major portion of the fluid
hydrocarbon feed stream comprises at least about two-thirds of said
stream.
3. The process of claim 2 in which said major portion comprises
from about 70% to about 80% by volume of said fluid feed
stream.
4. The process of claim 1 in which said hydrocarbon feed comprises
a natural gas stream.
5. The process of claim 1 in which said hydrocarbon feed comprises
light naphtha.
6. The process of claim 1 in which said hydrocarbon feed comprises
propane and butane.
7. The process of claim 3 in which the mole ratio of steam to
hydrocarbon feed in the first primary reforming zone is from about
2/1 to about 4/1, and said mole ratio in the second primary
reforming zone is from about 3/1 to about 6/1.
8. The process of claim 7 in which said hot reformer tube effluent
of the first primary reforming zone is at a temperature of from
about 800.degree. C. to about 900.degree. C., said effluent from
the second primary reforming zone is at a temperature between
700.degree. C. and about 860.degree. C., and the partially cooled
combined reformer gas stream is withdrawn from the second primary
reaction zone.
9. The process of claim 1 in which the partially cooled combined
reformer effluent stream is passed to a waste heat recovery zone
for the generation of steam therein.
10. The process of claim 9 in which steam generated in said waste
heat recovery is employed as said steam used in said catalytic
steam reforming operations.
11. The process of claim 9 and including passing said partially
cooled combined reformer effluent stream to a carbon monoxide shift
reactor zone.
12. The process of claim 10 in which import steam is employed for
said catalytic steam reforming operations in addition to the steam
generated in said waste heat recovery zone.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the steam reforming of fluid hydrocarbons.
More particularly, it relates to an improved process and apparatus
for reducing the fuel consumption and waste heat requirements of
steam reforming operations.
2. Description of the Prior Art
In conventional steam reforming of fluid hydrocarbons, the feed
material is conveniently passed through catalyst-containing
vertically hanging reformer tubes maintained at an elevated
temperature by radiant heat transfer and/or by contact with
combustion gases in the furnace of the tubular reactor. The hot
reformer tube effluent may be passed to a waste heat recovery zone
for the generation of steam that can be used in the stream
reforming operations.
Such conventional operations are commonly carried out at
temperatures of from about 800.degree. C. to about 900.degree. C.
with a mole ratio of steam to hydrocarbon feed of from about 2/1 to
about 4/1. While such operations have been effectively carried out
in practical commercial operations, there is a genuine need in the
art for improved processes and apparatus to reduce the fuel
consumption and waste heat requirements of steam reforming
operations and to reduce the capital investment costs of such
operations. This need is particularly acute in light of the rapidly
increasing cost of fuel employed in such operations.
Various processing techniques and apparatus have, of course, been
disclosed in the art with respect to hydrocarbon reforming and
cracking operations. For example, the Bongiorno patent, U.S. Pat.
No. Re. 24,311, discloses conventional steam reforming with the use
of the exhaust gas from the gas turbine used to compress the
product synthesis gas as a combustion-supporting gas to heat the
primary reformer furnace that produces the synthesis gas. Orr, U.S.
Pat. No. 2,519,696, discloses a horizontally oriented tube cracking
furnace in which gases to be cracked pass through and are preheated
in horizontal tubes and exit therefrom into direct contact with hot
combustion gases. The resulting gas stream passes through cracking
tubes countercurrently to the feed gases. The invention is said to
provide a maximum thermal heat exchange relationship in which the
gases to be cracked are preheated before being subject to reaction
heat conditions, with the incoming feed likewise cooling the
reaction mixture so as to prevent undesirable side reactions. With
respect to high pressure cracking operations, Woebcke et al., U.S.
Pat. No. 3,910,768, teaches the desirability of permitting the
process fluid and the combustion gas to operate at essentially the
same pressure, thereby relieving the pressure differential on the
reactor tubes. Hanging reactor tubes are provided in the convection
section with processing fluids passing downward therein
countercurrent to the upward flow of flue gas.
A heat exchanger-tubular steam reformer is disclosed by Kydd, U.S.
Pat. No. 3,607,125, in which process gas passes downwardly through
an annular, catalyst-filled space between a metal liner and a
centrally located product tube and thereafter rises upward in said
tube. The reactor tube is hung vertically with the thermal stress
thereon being minimized since the lower end by the tube is not
connected to any supporting structure. Kydd discloses that the
direction of process flow through the apparatus can be reversed,
with the process gas entering the centrally located hanging tube
and exiting, after passage downward through the catalyst, in an
upward direction along the annular space between the tube and the
wall of the apparatus.
Such prior art developments illustrate the desire to effectively
utilize the available waste heat from reforming and cracking
operations and to reduce the fuel requirements of such operations.
There remains a need for further developments of this type,
however, particularly to reduce the need for waste heat recovery,
as by steam generation, in applications in which there is little
need for export steam.
It is an object of the present invention, therefore, to provide an
improved process and apparatus for the steam reforming of fluid
hydrocarbons.
It is another object of the invention to provide a steam reforming
process and apparatus capable of permitting desirable reductions in
the fuel consumption and waste heat requirements of such reforming
operations.
It is a further object of the invention to provide for the
effective utilization of the heat generated in the steam reforming
of fluid hydrocarbons.
With these and other objects in mind, the invention is hereinafter
described with reference to particular embodiments thereof, the
novel features of which are particularly pointed out in the
appended claims.
SUMMARY OF THE INVENTION
The fluid hydrocarbon feed stream to a steam reforming operation is
divided into two streams, with a major portion thereof passing to a
conventional primary tubular reformer and with the hot reformer
tube effluent therefrom being used to supply heat for the reforming
of the remaining portion of the feed in a primary
reformer-exchanger unit. For this purpose, said hot effluent is
combined with the hot reformed gas passing from the primary
reformer tubes of the reformer-exchanger unit, and the combined hot
reformer gas steam is passed on the shell side of said
exchanger-reformer unit countercurrently to the flow of feed
material in the reformer tubes of said unit. Alternately, the hot
effluent from the conventional primary reformer can be passed to a
secondary reforming zone with the effluent from said secondary zone
being combined with said hot reformed gas from the primary
reformer-exchanger unit. By such process and apparatus, the fuel
consumption requirements of the overall steam reforming operation
are substantially reduced and the recoverable waste heat in the
overall reformer effluent is likewise reduced as compared with
conventional tubular reforming of hydrocarbon feed streams. Thus,
less export steam is produced for use outside the reforming
operation. In larger size plants, the capital investment costs for
the steam reforming operation can be reduced. The
reformer-exchanger portion of the overall reforming apparatus is
advantageously an internally insulated reformer-exchanger unit in
which the inner wall of the shell side of said reformer-exchanger
can be lined with refractory material or in which a double shell
configuration can be employed with means for passing water or a
portion of the feed material between the inner and outer shells of
said reformer-exchanger unit.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is hereinafter described with reference to the
accompanying drawing illustrating the process flow and apparatus
employed in the practice of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The invention enhances the overall technical-economical feasibility
of steam reforming operations. The objects of the invention are
accomplished by dividing a fluid hydrocarbon stream into two
portions, one of which is subjected to conventional steam reforming
with the hot effluent therefrom supplying heat for the reforming of
the remainder of said stream in a novel reformer-exchanger system
as described herein. The fuel consumption requirements for steam
reforming operations are thereby significantly improved. The waste
heat requirements of such operations are also significantly
reduced, providing an advantage that is particularly significant in
applications in which there is little or no need for export steam.
The invention thus achieves appreciable savings in operating costs
compared to conventional operations. In particular embodiments,
usually in larger size applications, such savings are accompanied
by investment cost savings that further enhance the overall
desirability of the invention for practical commercial steam
reforming operations.
The catalytic conversion of hydrocarbons by reaction with steam at
elevated temperature is, of course, well known in the art. In this
process, a fluid hydrocarbon, such as natural gas, is converted to
a hot reformed gas mixture containing principally hydrogen and
carbon monoxide according to the reaction;
known generally as primary reforming and widely used in the
production of synthesis gas or pure hydrogen. The primary reforming
reaction is endothermic in nature, and the conventional operation
is usually carried out by passing a gaseous mixture of fluid
hydrocarbon and steam through an externally heated reaction tube or
group of tubes. The tubes are packed with a suitable catalyst
composition, such as solid catalyst granules deposited on an inert
carrier material. The resulting reformed gas mixture discharges
from said tubes as a hot reformer tube effluent from which heat may
be recovered in a waste heat recovery zone. The hot reformer tube
effluent from primary steam reforming is often passed, however,
directly to a secondary reforming zone in which the reformed gas
mixture is passed, together with oxygen or air, through a bed of
the reforming catalyst so that said oxygen or air reacts with
unconverted methane present in the reformed gas mixture. The gas
mixture from such secondary reforming can then be cooled in said
waste heat recovery zone prior to further processing.
Such conventional processing is employed in the practice of the
present invention for the treatment of a major portion of the fluid
hydrocarbon feed stream being passed to a steam reforming
operation. The heat required for such conventional primary
reforming is usually supplied by burning a fluid hydrocarbon fuel
with air in the primary reforming zone external to the
catalyst-filled reformer tubes positioned therein. The reformer
tubes are thus heated and maintained at an elevated temperature by
radiant heat transfer and/or by contact with the hot combustion
gases produced by such burning of hydrocarbon fuel.
In accordance with the present invention, the remaining portion of
the fluid hydrocarbon feed stream is catalytically reacted with
steam at an elevated temperature in a second primary reforming zone
that likewise has catalyst-containing reformer tubes positioned
therein. The hot effluent from said reformer tubes comprises a hot
reformed gas mixture that is combined with the hot reformed gas
mixture from conventional primary reforming, or from the secondary
reforming thereof. The heat content of the combined gas steam is
employed to supply heat to maintain the second primary reforming
zone at an elevated temperature as further described with reference
to the illustrated embodiment of the reformer-exchanger system and
overall steam reforming apparatus and process flow shown in the
drawing.
The conventional primary reforming zone of the invention is
represented in the drawings by the numeral 1. Vertically oriented
reformer tubes 2 are positioned therein, said tubes each containing
catalyst beds 3 comprising a suitable reforming catalyst material.
The fluid hydrocarbon feed stream in line 4 is divided into two
streams, with a major portion of the feed stream passing in line 5
to said primary reforming zone 1 together with steam shown as being
introduced through line 6 for mixing with said fluid hydrocarbon
feed stream in said line 5. The remaining portion of the
hydrocarbon feed stream is passed in line 7 to a second primary
reforming zone comprising primary reformer-exchanger 8. Steam from
line 9 is mixed with said remaining portion of the hydrocarbon feed
steam thus being fed to said reformer-exchanger 8. A fluid
hydrocarbon fuel, such as a side stream from the fluid hydrocarbon
feed stream 4, is passed through line 10 to the shell side of
primary reforming zone 1 for burning therein, as with air shown
generally as being fed to said zone 1 through line 11. Flue gas is
removed from zone 1 through line 12. Hot reformer tube effluent
comprising a reformed gas mixture, upon exiting from reformer tubes
2 in said primary reforming zone or unit 1, is passed through line
13 to primary reformer-exchanger 8 for the utilization of the heat
content of said hot effluent therein. In accordance with
conventional practice, it is within the scope of the invention to
pass the effluent from primary reforming directly to a secondary
reforming zone as noted above. In the drawing, therefore, secondary
reforming zone 14 is shown as an optional unit in said line 13
between primary reformer 1 and reformer-exchanger 8. Line 15 to
said secondary reformer 14 is to supply air or oxygen for reaction
with unconverted methane present in the reformed gas mixture from
primary reformer 1.
The hot effluent from primary reformer 1, or said effluent after
passing through secondary reformer 14, is passed through said line
13 to the hot discharge end 16 of reformer tubes 17 in
reformer-exchanger 8. In said hot discharge end, said hot effluent
from conventional reforming is combined with the hot effluent from
said reformer tubes 17, said latter hot effluent comprising a
reformed gas mixture formed upon passage of remaining portion 7 of
the hydrocarbon feed stream and steam from line 9 through said
reformer tubes 17 containing catalyst beds 18 therein. The heat to
maintain reformer-exchanger 8, and catalyst-containing reformer
tubes 17 therein, at an elevated temperature for the catalytic
reaction of said hydrocarbon feed stream and steam is supplied by
passing the hot combined effluent streams comprising a hot combined
reformer gas steam countercurrently on the shell side 19 of said
reformer-exchanger 8. In thus supplying heat to the reaction
mixture within said reformer tubes 17, the combined reformer gas
stream is partially cooled and is withdrawn from the hot discharge
end 20 of said reformer-exchanger 8, that comprises the second
primary reaction zone of the overall system, through hot discharge
line 21 as a reformed gas product stream. This product stream can
be pass through a conventional waste heat recovery zone, not shown,
for further cooling, as by the production of export steam, prior to
further conventional treatment or use, as for example in the
production of pure hydrogen from said combined reformed gas
mixture. Reformer-exchanger 8, thus employed to effectively utilize
the available heat of the conventional reforming effluent stream,
and of the hot effluent from the reformer tubes of said
reformer-exchanger itself, is desirably internally insulated as by
lining 22 with a suitable refractory material.
The term "fluid hydrocarbon," as used herein, is intended to
include not only normally gaseous hydrocarbons, such as natural
gas, propane and butane, but also pre-vaporized normally liquid
hydrocarbons, such as hexane or petroleum refining low-boiling
fractions such as naphtha. It will also be understood by those
skilled in the art that the catalyst employed in the practice of
the invention can be any one or more suitable reforming catalysts
employed in convention steam reforming operations. The metals of
Group VIII of the Periodic System having an atomic number not
greater than 28 and/or oxides thereof and metals of the left-hand
elements of Group VI and/or oxides thereof are known reforming
catalysts. Specific examples of reforming catalysts that can be
used are nickel, nickel oxide, cobalt oxide, chromia and molybdenum
oxide. The catalyst can be employed with promoters and can have
been subjected to various special treatments known in the art for
purposes of enhancing its properties. The composition and method of
preparation of the catalyst composition for use in the invention
form no part of the novel features of the invention and, therefore,
are not further described herein. Generally, however, promoted
nickel oxide catalysts are preferred, and the primary reformer
tubes of the reformer zones are packed with solid catalyst
granules, usually comprising such nickel or other catalytic agent
deposited on a suitable inert carrier material. As secondary
reforming is also a catalytic process, the hot reformer tube
effluent is commonly passed through a stationary bed of such
reforming catalyst in those embodiments in which secondary
reforming of the effluent from conventional primary reforming is
employed.
The conditions employed in the primary steam reforming operations
of the invention are those serving to promote substantial
conversion of the fluid hydrocarbon feed stream to hydrogen and
carbon monoxide. Thus, the hot reformer tube effluent from the
conventional tubular reformer, i.e. primary reforming zone 1 of the
drawing, is at a temperature of from about 800.degree. C. to about
900.degree. C. As the heat content of said hot effluent is used to
maintain the temperature in the reformer-exchanger unit, the
temperature of the hot effluent from the reformer tubes of said
reformer-exchanger, i.e. reformer-exchanger unit 8 of the drawing,
tends to be somewhat less than in zone 1, being commonly on the
order of from about 700.degree. C. to about 860.degree. C. The
partially cooled combined reformer gas steam withdrawn from the
reformer-exchanger unit will commonly be at a temperature of from
about 400.degree. C. to about 600.degree. C. When secondary
reforming of the reformed gas mixture from primary reformer 1 is
employed, the partial combustion reaction therein tends to increase
the temperature of the gas mixture, so that the hot effluent from
the secondary reforming zone will commonly be at a temperature of
from about 900.degree. C. to about 1200.degree. C.
Both the fluid hydrocarbon feed stream and the steam supplied to
the reforming operation of the invention are, consistent with
conventional practice, preferably preheated prior to entering the
primary reforming zones. The hydrocarbon feedstock is preheated up
to as high a temperature as is consistent with the avoiding of
undesired pyrolysis or other heat deterioration. Since steam
reforming is endothermic in nature and since there are practical
limits to the amount of heat that can be added by indirect heating
in the reforming zones, preheating of the feed facilitates
attainment and maintenance of suitable temperature therein. It is
commonly preferred to preheat both the hydrocarbon feed and the
steam to a temperature of at least 400.degree. C. As indicated
above, the portion of the hydrocarbon feed passed to a conventional
tubular reformer, together with steam, contacts a steam reforming
catalyst preferably disposed in a plurality of furnace tubes that
are disposed so as to be maintained at an elevated temperature by
radiant heat transfer and/or by contact with combustion gases.
Fuel, such as a portion of the hydrocarbon feed, is burned in the
reformer furnace to externally heat the reformer tubes and to
supply the endothermic heat of reforming consumed therein. In the
reformer-exchanger employed for the steam reforming of the
remainder of the hydrocarbon feed, the heat content of the combined
reformer gas stream is employed to supply the endothermic heat of
reaction for the reformer-exchanger unit.
The ratio of steam to hydrocarbon feed will vary, as is know in the
art, depending on the overall conditions employed in each primary
reforming zone. The amount of steam employed is influenced by the
requirement of avoiding carbon deposition on the catalyst and by
the acceptable methane content of the effluent at the reforming
conditions maintained. On this basis, the mole ratio of steam to
hydrocarbon feed in the conventional primary reformer unit is
preferably from about 2/1 to about 4/1, while the mole ratio in the
primary reformer-exchanger unit is preferably from about 3/1 to
about 6/1. The higher amounts of steam commoly employed in the
reformer-exchanger unit are, at least in part, to compensate for
the generally lower reaction temperature maintained in the reaction
tubes of the reformer-exchanger than pertains in the reactor tubes
of the conventional tubular reformer.
It will be appreciated that steam reforming operations, including
those of the present invention, are commonly carried out at
superatmospheric pressure. The specific operating pressure employed
is influenced by the pressure requirements of the subsequent
process in which the reformed gas mixture or hydrogen is to be
employed. Although any superatmospheric pressure can be used in
practicing the invention, pressures of from about 350 to about 700
p.s.i.g. are commonly employed, although pressures of from about
175 to about 300 p.s.i.g., below 175 p.s.i.g., and up to as high as
1,000 p.s.i.g. can be maintained in particular embodiments of the
invention.
The present invention is carried out by catalytically reacting a
major portion of the fluid hydrocarbon feed stream with steam in a
conventional tubular reformer, catalytically reacting the remaining
portion of said hydrocarbon feed stream with steam in the
reformer-exchanger portion of the overall process and apparatus
herein described and claimed, and utilizing the hot combined
reformer gas stream to supply heat to maintain the elevated
temperature employed in the reformer-exchanger unit. Those skilled
in the art will appreciated that the precise amount of the overall
hydrocarbon feed passed to each of said primary reforming zones
will depend upon the particular conditions applicable in any given
application, including the nature of the hydrocarbon feed, the
catalyst employed, the steam/hydrocarbon ratio, the temperature and
pressure of the reaction and the like. In general, however, said
major portion of the hydrocarbon feed thus passed to the
conventional primary reformer will generally comprise at least
about two-thirds of the overall hydrocarbon feed stream. In
preferred embodiments, said major portion of the hydrocarbon feed
will comprise from about 70% to about 80% by volume based on the
overall hydrocarbon feed to the steam reforming operations of the
invention.
In the illustration of the drawing, both primary reformer 1 and
reformer-exchanger 8 are shown with vertically oriented tubes
although it will be appreciated that horizontally oriented tubes
can also be employed in the practice of the invention. The use of
hanging tubes is particularly desirable in the reformer-exchanger
unit as the hot effluent from the hanging reformer tubes, following
steam reforming during downward passage of the steam-hydrocarbon
mixture in the hanging tubes, can conveniently be combined with the
hot reformer tube effluent from conventional reforming at the hot
discharge end of said hanging reforming tubes in the lower portion
of reformer-exchanger 8. The combined reformer gas stream thus
formed is thereupon passed upward on the shell side of the
reformer-exchanger, countercurrently to the flow of the reaction
mixture in the reformer tubes, to provide the necessary heat to
maintain the elevated temperature in the reformer tubes. As the
pressure inside and outside the hanging tubes is essentially the
same, tube rupture is avoided without the necessity for incurring
undue costs in this regard. For preferred operation, the primary
reformer-exchanger unit comprises an internally insulated
reformer-exchanger unit or zone. For this purpose, the inner wall
of the shell side of the reformer-exchanger can be lined with MgO
or other convenient refractory material so as to protect the outer
shell of the unit and to effectively utilize the available heat of
the combined reformer effluent stream. Alternately, the internally
insulated reformer-exchanger unit can comprise a double shell unit
with means for passing water or a portion of the feed material, or
other coolant fluid between the inner and outer shells of said
reformer-exchanger unit.
The partially cooled combined reformer effluent stream withdrawn
from the reformer-exchanger unit is desirably passed to a
conventional waste heat recovery zone for the recovery of at least
a portion of its remaining heat content prior to further downstream
processing or use. The heat content of said partially cooled stream
can be used for the generation of steam, for example, with said
steam being conveniently employed as process steam for the
catalytic steam reforming operations of the invention. Because of
the effective use of the heat content of the combined reformer gas
stream in the reformer-exchanger, the amount of steam recovered in
the waste heat recovery zone is significantly less than is
recovered in conventional operations. Because of the generally
higher steam requirements of the reformer-exchanger as compared
with the higher temperature, conventional primary tubular reformer,
the amount of excess or export steam to be withdrawn from the
overall reforming operation is thus considerably less than the
amount of export steam generated in conventional reforming
operations. This is an important advantage where there is little or
no need for such export steam, as where other, lower cost steam is
available in the facilities of which the reforming operations are a
part. It will also be appreciated by those skilled in the art that,
in some embodiments of the invention, there might actually be a
deficiency of steam generated in the practice of the invention, so
that low cost steam generated outside the process of the invention
is imported for use therein. Again, this constitutes an important
advantage of the invention in applications where there is no need
for export steam from the steam reforming operations.
The invention is further described with reference to particular
embodiments illustrated by the following examples.
EXAMPLE 1
In a steaming reforming operation to produce a reformed gas mixture
for the production of 1.06.times.10.sup.6 SCF (Standard Cubic Feed)
of hydrogen/hour based on plant operations of 8,000 hours/year,
naphtha is employed as the hydrocarbon feed material and as the
fuel for the conventional primary tubular reformer. By use of the
apparatus and process of the present invention, without employment
of a secondary reforming unit, appreciable savings in operating
costs are achieved as compared with a conventional primary
reforming operation of the same capacity using the same fuel/feed
material. The hot reformer tube effluent from conventional
reforming in a tubular reformer is removed from the reformer tubes
thereof at about 830.degree. C. for passage to a boiler or other
waste heat recovery zone. The partially cooled combined reformer
effluent removed from the reformer-exchanger unit of the invention,
on the other hand, is at a temperature of about 650.degree. C.,
thus appreciably reducing the waste heat requirments as compared to
conventional reforming operations. The composition of the reformed
gas mixture removed from the reformer-exchanger of the invention is
nearly the same as that removed from the corresponding conventional
reforming operations. In this embodiment of the invention, about
77% by volume of the overall naphtha feed material is passed to the
conventional primary reformer and about 23% is passed to the
reformer-exchanger, apart from feed material employed for fuel. The
hot reformer tube effluent from the primary reformer portion of the
apparatus of the invention is at about 860.degree. C. and is passed
directly to the lower portion of the reformer-exchanger for
combination with the effluent at the hot discharge end of the
reformer tubes thereof. In addition to the reduced steam production
resulting from the use of the heat content of the hot combined
reformer gas stream in the reformer-exchanger, the fuel consumption
in the practice of the invention is also considerably reduced. A
reduction in export steam production of 40,455 lbs/hr is achieved
by the invention, resulting in a lower credit for export steam of
$101.14/hr based on a steam value of $2.50/1000 lbs. The
consumption of naphtha fuel is reduced by 2,375 lbs/hr by the
practice of the invention resulting in a cost savings of $197.99/hr
based on a fuel value of $0.50/gallon or 8.34.cent./lb. While the
savings in net operating costs by use of the present invention will
vary with the values assigned to steam and fuel, it will be seen
that the invention achieves a substantial reduction in hydrocarbon
fuel consumption in addition to a reduction of recoverable waste
heat in the reformer effluent compared with conventional
operations. In applications where there is no demand for export
steam, therefore, the present invention will always provide a
significant savings in operating cost as compared to conventional
operations.
The savings in operating cost illustrated above is directly
proportional to plant capacity. In addition, it has been determined
that, for the same productive capacity, the use of the apparatus of
the invention will result in a reduction in the number of tubes in
the conventional fired reformer furnace. The flue gas ducts will
likewise be reduced, with both such reductions estimated at about
20%. The flue gas fan, combustion air blower and other related
equipment can be employed at smaller capacity when the primary
reformer is combined with a reformer-exchanger in the practice of
the invention. Although the reformer-exchanger unit represents an
additional piece of equipment, it should be noted that it replaces
a part of the steam producing heat exchange area in the
conventional processing arrangement. For these reasons, the savings
in investment cost can be quite significant for units of larger
capacity, such as the plant of the illustrative example. This
advantage will diminish with smaller capacities, but even at about
1/10 said capacity of the example, the investment costs of the
invention will be smaller than, or approximately equal to, the
investment cost of the conventional reforming unit.
EXAMPLE 2
In this embodiment, the reformer-exchanger of the invention is
employed in the steam reforming of a methane feed to produce a
reformed gas mixture that is passed to a conventional CO shift to
form additional hydrogen and is thereafter forward to a pressure
swing adsorption unit for the production of pure hydrogen. For the
purpose, 1,000 moles/hour of a methane feed stream at 24 bar, i.e.
348 psia, is heated to 400.degree. C. and desulphurised on a ZnO
bed. The feed stream is then divided into two streams, with 770
moles/hour being mixed with 2,080 moles/hour of steam and
introduced to the utalytic tube of a directly fired primary steam
reformer wherein it is heated to a reaction temperature of
865.degree. C. The remaining portion of the feed stream, i.e. 230
moles/hour of methane, is mixed with 920 moles of steam and
introduced to the catalyst tubes of the reformer-exchanger wherein
it is heated to a reaction temperature of 770.degree. C. The
effluent from the primary reformer is mixed with the effluent at
the discharge end of the catalyst tubes of the reformer-exchanger.
The resulting hot combined reformer gas stream has a temperature of
840.degree. C. and is cooled to 580.degree. C. sharing its passage
on the shell side of reformer-exchanger countercurrently to the
passage of the reaction mixture in the catalyst tubes of said
reformer-exchanger. The thus-cooled combined reformer effluent
exiting from the reformer-exchanger is further cooled to
310.degree. C. and is passed to a conventional CO shift reaction
zone in the catalytic bed of which most of the carbon monoxide
present in the combined reformer effluent is reacted with steam to
form additional hydrogen and carbon dioxide. The effluents from the
primary reformers and from the CO shift are summarized in Table 1
as follows:
TABLE 1 ______________________________________ Direct Fired
Reformer- Combined After Reformer Exchanger Effluent CO-Shift
______________________________________ Temperature, .degree.C. 865
770 840 370 Pressure, bar 20.7 20.7 20.7 20 Moles/hour CH.sub.4
138.6 66.7 205.3 205.3 CO 415.8 69 484.8 130 CO.sub.2 215.6 94.3
309.9 654.7 H.sub.2 O 1232 662.4 1894.4 1539.6 H.sub.2 2109.8 584.2
2694.0 3048.8 Total moles/hour 4111.8 1476.6 5588.4 5588.4
______________________________________
Following Co-shift, the process gas is further cooled, and more
steam is generated. A total of 1800 kgmol/hour of steam is produced
in the process gas coolers. After cooling to ambient temperature,
the gas is separated in a pressure swing adsorption unit to 2622
kgmoles/hour of pure hydrogen and a waste gas with a total heat
content of 304 GJoule/hour, based on lower heating value. The waste
gas is used as fuel for the direct fired reformer.
The heat balance for this illustrative example of the use of the
reformer-exchanger of the invention is compared with that of
conventional prior art primary steam reforming in Table 2
below.
TABLE 2 ______________________________________ Heat Balance (All
values expressed in GJoule/hour) Invention Prior Art
______________________________________ Feed Methane* 800 792 Fuel
Methane* 40 120 Total-Heat In Feed plus Fuel 840 912 Steam Export
(generated-used) 10 75 Heating Value of Product Hydrogen* 632 632
Heat Losses- Stack and Process Gas Cooler 198 205 Total-Heat Chart
840 912 Efficiency, % (Hydrogen only) 75.2 69.3 Efficiency, %
(Hydrogen plus steam export) 76.4 77.5
______________________________________ *Low Heating Value
It will be appreciated that, frequently, no export steam is needed
in the overall processing operation. The Example illustrates that,
in the practice of the invention, the desired steam reforming
operation is accomplished at a substantial, i.e. 8%, reduction in
the feed plus fuel requirements of the steam reforming
operation.
EXAMPLE 3
In this illustrative examples, 1000 moles/hour of methane feed at
31 bar is divided into two streams, one of which is passed to a
conventional primary reformer and then to a secondary reforming
zone. The effluent from the secondary zone is combined with the
reformed gas mixture discharged at the hot discharge end of the
catalyst-containing reformer tubes of the reformer-exchanger to
form a combined reformer gas stream that is passed on the shell
side of said reformer-exchanger. Thus, 750 moles/hour of methane is
mixed with 2025 moles/hour of steam and is heated in the direct
fired primary reformer to 815.degree. C. and is thereafter passed
to a secondary reformer in which 150 moles/hour of oxygen are added
thereto. The outlet temperature of the secondary reforming zone is
940.degree. C. The second portion of the feed material, i.e. 250
moles/hour of methane, is mixed with 1900 moles/hour of steam and
is heated in the reformer-exchanger to 840.degree. C. The effluent
compositions of the various processing steams are as set forth in
Table 3 below.
TABLE 3 ______________________________________ Direct Fired
Secondary Reformer- Combined Reformer Reformer Exchanger Effluent
______________________________________ Temperature, .degree.C. 815
940 840 916 Pressure, bar 29 28.2 28.2 28.2 Moles/hour CH.sub.4 270
52 53 105 CO 263 465 107 572 CO.sub.2 217 233 90 323 H.sub.2 O 1328
1394 613 2007 H.sub.2 1657 2027 678.5 2705.5 Total 3735 4171 1541.7
5712.5 ______________________________________
In this example, 25% of the feed is being reformed in the
reformer-exchanger unit. The reformer tube effluent from the direct
fired primary reformer is first passed to a secondary reformer
operated with oxygen addition. It will be understood that preheated
air can be employed in the secondary reformer instead of oxygen,
this modification being particularly advantageous when ammonia
syngas is to be produced instead of pure hydrogen. The more oxygen,
or air, that is added to the secondary reformer, the larger can be
the portion of the original feed stream that is passed to the
reformer-exchanger. It should also be noted that if sufficient
oxygen is available for use in the secondary reformer, the size of
the direct fired primary reformer can be greatly reduced, with much
of the feed gas passing through the primary reformer to the
secondary reformer for conversion therein.
The invention is of practical commercial interest, therefore,
because of its savings in operating costs and its potential for
savings in investment costs particularly for large plants. In the
absence of a demand for steam, the invention will enable
appreciable savings in operating costs to be realized, the
desirable reduction in the waste heat requirements of the invention
complementing the significant reduction in fuel consumption
achieved by the invention. The steam reforming operations of the
invention represent, therefore, a very desirable advance in the
reforming art, enhancing the technical and economic feasibility of
such operations at a time of increased costs and a growing desire
to achieve reductions in fuel consumption and other costs
associated with standard commercial operations.
* * * * *